feat(todo): tasks dragged into place or under another, the add button dropped where the task goes (ADR 0033, D16)

- A project's row, held, is dragged before or after another, or a
  little to the right to become its sub-task, or onto a section's
  heading; its sub-tasks go with it, and Order changed offers Undo
  (TaskRepository.place). The list scrolls at its edges.
- The add button is dragged onto a project's list to add the task
  between two rows, onto a day in Upcoming for that day, onto Today
  for today; dropped back on its place, an ×, it gives up.
- Holding a row now drags it; a swipe to the right selects it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/todo/halkyon_todo_flutter/test/feature/task/task_repository_test.dart

@@ -177,6 +177,62 @@ void main() {
     },
   );
 
+  test('a task placed elsewhere in its project takes its sub-tasks into its '
+      'section; under another task it becomes its sub-task; restoring what '
+      'it returns undoes it', () async {
+    final tasks = await device(alice);
+    final pack = await tasks.add(
+      const TaskContent(title: 'Pack', projectId: 'inbox', position: 1),
+    );
+    final socks = await tasks.add(
+      TaskContent(title: 'Socks', projectId: 'inbox', parentId: pack),
+    );
+    final trip = await tasks.add(
+      const TaskContent(title: 'Trip', projectId: 'inbox', position: 2),
+    );
+
+    final before = await tasks.place(pack, sectionId: 's', position: 0.5);
+    var all = {for (final t in await tasks.all()) t.id: t.content!};
+    expect(all[pack]!.sectionId, 's');
+    expect(all[pack]!.position, 0.5);
+    expect(all[pack]!.parentId, isNull);
+    expect(all[socks]!.sectionId, 's', reason: 'its sub-task along');
+    expect(all[socks]!.parentId, pack);
+    expect(all[trip]!.sectionId, isNull, reason: 'the others stay');
+
+    await tasks.place(trip, sectionId: 's', parentId: pack, position: 3);
+    all = {for (final t in await tasks.all()) t.id: t.content!};
+    expect(all[trip]!.parentId, pack);
+    expect(all[trip]!.sectionId, 's');
+
+    await tasks.restore(before);
+    all = {for (final t in await tasks.all()) t.id: t.content!};
+    expect(all[pack]!.sectionId, isNull);
+    expect(all[pack]!.position, 1);
+    expect(all[socks]!.sectionId, isNull);
+  });
+
+  test('a task is never placed under itself or its own sub-task', () async {
+    final tasks = await device(alice);
+    final pack = await tasks.add(
+      const TaskContent(title: 'Pack', projectId: 'inbox'),
+    );
+    final socks = await tasks.add(
+      TaskContent(title: 'Socks', projectId: 'inbox', parentId: pack),
+    );
+
+    expect(
+      () => tasks.place(pack, sectionId: null, parentId: socks, position: 0),
+      throwsArgumentError,
+    );
+    expect(
+      () => tasks.place(pack, sectionId: null, parentId: pack, position: 0),
+      throwsArgumentError,
+    );
+    final all = {for (final t in await tasks.all()) t.id: t.content!};
+    expect(all[pack]!.parentId, isNull, reason: 'nothing written');
+  });
+
   test('moving tasks to a project takes their sub-tasks along, out of any '
       'section, and a sub-task moved without its parent becomes a task of '
       'its own', () async {